Here, as mentioned above, we allow for the drift potential \(\phi \) , and thus \(\Phi \) , to be multi-valued. The argument then that \(\Psi \)  be single-valued is addressed in [1]}. In short, once one has a linear Schrödinger equation, and therefore, a superposition principle, we must require that these additional ingredients remain consistent with the probabilistic structure of ED. This, it turns out, is sufficiently constraining — for the probability \(\rho = | \Psi |^{2}\)  to remain single valued, the wave function \(\Psi \)  must be either single-valued or double-valued.
